Electromagnetic and other waves may be used to transmit and receive information through modulated waveforms. For example, radar may use transmitted and reflected modulated waveforms to determine an object's location. Cellular telephones and digital television broadcasts all use complex modulation schemes (such as Gaussian minimum shift keying (GMSK) and orthoganol frequency division multiplexing (OFDM), respectively) to reliably transmit information in a variety of forms (e.g., voice, pictures, video and digital data). Reliable information transmission, for example with an acceptable bit error rate in the context of digital signals, may be difficult to achieve, however, because of many problems, including narrow band and multipath fading, narrow band interference, etc.
Information transmission is often segmented into physical frequency channels. Certain characteristics may be discrete to a given channel, and these channel characteristics primarily determine the rate at which data can be transmitted and the power required to maintain a certain fidelity (e.g., bit error rate) of the communication. Thus, a modulation scheme may be fine tuned to a particular channel, and so more efficiently transmit information within that channel. Again, however, fading and interference may occur within a channel and it is the characteristics and the behavior of the modulation scheme under differing channel characteristics that determine how effective a modulation scheme is.
Thus, there remains a need for methods and apparatus for transmitting and receiving information via modulated waveforms that minimize fading and interference problems.
In addition, certain military applications exist where covert transmission and a transmitted waveform that is both difficult to detect and decode/demodulate are an advantage, and for which an unmet need remains.